Thin coating characterization by Rayleigh waves: An analytical model basedon normal-mode theory

The acousto-optic technique to evaluate coated substrates, presented by Dev older et al. (Appl. Phys. Lett. 68, 1732 (1996)], uses the phase dependence of the reflected beam on the coating parameters. In this theoretical paper , a fully analytical model, based on normal-mode theory, is applied to this problem. It gives a transparent expression for the phase shift between the reflected and the incident field due to the presence of thin coatings (coa ting thickness much smaller than wavelength sound). The known numerical res ults from Fourier simulations, i.e. the phase dependency on the coating par ameters and the advantages of using higher frequencies and wider beams, are reproduced. However, thanks to the analytical nature of the new theory, ad ditional results are obtained. The calculation time is reduced by two order s of magnitude, which is important for the inverse problem. A technique for absolute thickness measurements is proposed. Measuring more to the right o f the second reflected maximum increases the phase sensitivity. The versati lity of the normal-mode approach is illustrated by applying it to other non destructive testing (NDT) applications, such as hardness determination and adhesion testing, which are briefly discussed. (C) 2001 Acoustical Society of America.